1. Field of the Invention
The invention relates to sensing temperature, pressure and levels of liquids with particular application to fuel tanks. The invention further involves remote sensing of such physical phenomena if required.
2. Description of the Prior Art
There are environments wherein it is desirable to sense various physical parameters such as pressure, temperature, or liquid level where electrically based sensors or sensors containing metal may present a hazard from sparks. For example, it is often desirable to sense such parameters in fuel or other tanks where volatile fluids or gases are present. Sensors for use in such tanks should be non-electrical, non-static, non-spark producing and non-conducting. Such sensors should also accurately measure the desired parameter, be of rugged construction and be relatively inexpensive. Specifically, it is desirable to measure such parameters in marine vessel fuel tanks. Fires in large oil tankers have been caused by malfunctioning or damaged electrical temperature and pressure sensors in the fuel tanks thereof.
Measurement of pressure provides data from which the fuel level in the tank can be derived. One pressure sensor can measure liquid height in a tank if the liquid density is known. Two sensors are required for liquid level measurement if the density is unknown. The levels of two immiscible fluids in a tank can be measured by a string of pressure sensors. This condition arises on marine vessels which replace pumped fuel oil, such as JP-5, with sea water to provide ballast. Such a liquid level sensor system may be implemented by multiplexing several pressure sensors into a vertical array. It is appreciated that modern aircraft carriers include over 500 fuel tanks so that arrays of expensive pressure sensors would be prohibitively costly and, because conventional sensors would involve electricity, prohibitively dangerous.
Fiber optic sensors need not be electrical or metallic. The prior art, however, does not teach or suggest the use of an entirely satisfactory fiber optic sensor for the described purposes. Generally, three types of fiber optic sensors are known for measuring such parameters; viz, micro-bending sensors, interferometric sensors and polarization type sensors.
Fiber optic micro-bending sensors are designed to sense pressure by excluding light from the fiber in proportion to changes in pressure. The phenomenon is denoted as micro-bending loss. The output light intensity levels from such sensors diminish with increase in measured pressure as pressure is transduced into light loss. Since the accuracy of the measurement diminishes because of the lower light levels, the dynamic range of such sensors is severely limited. With the micro-bending sensor, changes in pressure are sensed by changes in output light levels. Such light level changes may also be caused by bending of the input or return cables and variations in source intensity caused by aging, power supply fluctations or drift and the like. Such changes in output light levels are inseparable from changes in the measured parameter. These disadvantages also maintain when micro-bending sensors are utilized to sense temperature. The micro-bending sensor tends to be a delicate readily damaged sensor.
Interferometeric fiber optic sensors measure changes in pressure by applying pressure to an optical fiber changing the index of refraction thereof. This phenomenon results in a phase delay which is measured by utilizing a Mach-Zehnder or Michaelson interferometer configuration. Although interferometric sensors are extremely sensitive, they are also extremely expensive costing many thousands of dollars for each sensor. Thus, in applications requiring thousands of sensors, the interferometeric sensor approach would be prohibitively expensive. Such sensors are extremely complicated and difficult to utilize outside of a laboratory environment. Interferometric sensors utilized to measure temperature would also suffer from the same shortcomings. Interferometric sensors require sophisticated modulation techniques which render the sensors unsuitable for the described applications.
Polarization varying fiber optic sensors alter the polarization state of a polarized optical signal in accordance with a change in temperature or pressure. Such polarized light sensors require special optical fiber and expensive polarizing beamsplitters. Additionally, perturbations in input or output leads or in the source cannot be readily separated from true changes in temperature or pressure.
Additionally, fiber optic temperature sensors based on the temperature dependent emission spectra of rare earth phosphors are known in the art. Such sensors are excessively expensive and require very complicated electro-optical signal processing. Distributed temperature sensors are also known which measure the temperature dependent anti-Stokes Raman backscattering via optical time domain reflectometry. Such sensor configurations are extremely expensive and are only practical when a large number of distributed temperature measurements are required. Such a sensor approach is inappropriate where single point temperature measurements are required.
A variable ratio fiber optic coupler sensor is described in U.S. Pat. No. 4,634,858, issued Jan. 6, 1987 entitled "Variable Coupler Fiber Optic Sensor" by Gerdt et al and assigned to the assignee of the present invention. Said U.S. Pat. No. 4,634,858 is incorporated herein by reference. Although the sensor of said U.S. Pat. No. 4,634,858 provides numerous advantages over the sensors hereinabove described, the variable ratio fiber optic coupler sensor has not heretofore been utilized or configured to resolve the problems of the above-described applications.